Osteoporosis and osteopenia affect over 200 million people worldwide and result in $14 billion in health care costs annually in the US alone. With an increasing aging population it is estimated that the 67 million Americans will be afflicted by low bone mass by the year 2020. Post-menopausal women are by far the largest population that suffers from osteoporosis. Current treatments to prevent bone loss predominantly utilize pharmacologic agents that target osteoclast activity, but their long-term use can have unwanted side effects including increased risk of unusual fractures. Thus physicians and their patients are looking for novel prevention and treatment options. Available data support a role for pro-inflammatory cells and cytokines as therapeutic targets because of their role in mediating osteoclast maturation and activity during menopause. Intestine-bone signaling is demonstrated to contribute to the regulation of bone density, yet very little is known about how menopause impacts the intestine and the role that this could play in bone loss. We have identified, for the first time that estrogen deficiency (induced by ovariectomy, ovx) leads to intestinal inflammation and causes dysbiosis of the intestinal microbiota in mice. Most importantly, oral administration of a human derived probiotic, Lactobacillus reuteri ATCC PTA 6475, inhibits ovx induced intestinal changes, bone marrow T-cell changes and prevents bone loss in ovx mice. Based on our novel findings, we hypothesize that L. reuteri prevents ovx induced bone loss by reducing gut and bone inflammation. The following aims are proposed to elucidate the role of gut-bone interactions in ovx induced bone loss as well as the mechanisms of L. reuteri mediated microbiota changes that prevent osteoporosis. AIM 1. Investigate the role of the intestinal microbiota in ovx induced bone loss. AIM 2. Identify the host mechanisms by which L. reuteri 6475 prevents ovx induced bone inflammation and bone loss. AIM 3. Establish the mechanistic basis of L. reuteri 6475 prevention of osteoporosis in ovx. Outcomes of our studies could provide a paradigm shift in understanding the role of gut-bone signaling axis in menopause and the role of estrogen in shaping our intestinal microbiota. Our findings will also provide the foundation for future clinical studies to determine the effectiveness of probiotics to maximize bone health in post-menopausal women.